This invention relates generally to medical imaging systems, and more particularly, to methods and systems for calibrating a Computed Tomography (CT) system.
CT systems may be used for imaging tissues and bones in various body parts. A CT system generally includes an image acquisition component, a processor and a controller. The image acquisition component has an X-ray source and a detector. Each detector includes a plurality of elements. Each element includes a scintillation crystal and a photosensor. Some examples of photosensors include photodiode pixels and phototransistors. The photosensors are broadly based on two kinds of technologies, namely, front-illuminated and back-illuminated. The two technologies are mainly distinguished by whether the electrical signal to the photosensors is routed from the front or the rear of the photosensors.
The X-ray source directs X-ray beams through the object being scanned. These X-ray beams are attenuated by the object and are absorbed by the detector. The detector converts the X-ray radiation energy into electric signals. Ideally, the cumulative electric signal collected by the detector is linearly related to X-ray radiation energy that is absorbed by the detector. This criteria is fulfilled when requirements, such as, but not limited to, stability of the detector over time and temperature, sensitivity to focal spot motion and light output over life, are met. The electrical signals absorbed by the detector are reconstructed by the controller to form a CT image of the object.
With advancements in technology in CT systems, there has been a growing need for more precise calibrations and alignments. The calibrations in CT systems are used to remove artifacts from CT images. Artifacts such as smudges and center spots in CT images may be caused by crosstalk between the photosensors and/or the scintillation crystals (in a detector).
In a detector using back-illuminated photosensors, the elements of the detector are connected and aligned in two directions, for example, the x and z directions. The close proximity of the photosensors causes crosstalk in both these directions. With the higher number of slices in multi-slices detectors and thin slices, the crosstalk becomes an important parameter to optimize and control. The crosstalk in both x and z directions include an average crosstalk and a module-to-module crosstalk. The module-to-module (channel-to-channel) crosstalk may be attributed to non-identical relative behavior in detector elements (module). The non-identical relative behavior includes factors, such as, fluctuation in the scintillator behavior from one scintillator element of the detector to an adjacent scintillator element and the lateral diffusion of photocarriers in the material of the photosensors. The lateral diffusion of photocarriers in the material of the photosensors causes photosensor electrical crosstalk. This results in photocarriers diffusing out of the photosensor collection junction area in which the photocarriers are generated. These photocarriers are then collected by adjacent or neighboring photosensors. This effect is more likely in back-illuminated photosensors than in front-illuminated photosensors, as the thickness of the back-illuminated photosensors increases the diffusion length before the photosensor collection junction.
Module-to-module z-crosstalk is difficult to correct because of the photosensor alignment requirements of the back-illuminated photosensor. In case the scintillator cell is not aligned (or centered) to the diode active area, the crosstalk from that cell to its neighboring cell will be unbalanced and will cause image artifact when slope anatomy is imaged. At the same time, the amount of crosstalk coming from the neighboring cell to the non-centered cell will be unbalanced when a slope anatomy is imaged.
Further, the z-crosstalk error is one of the factors that cause smudges and band artifacts in CT images obtained for objects that change in thickness sharply in the z-direction.
Some known methods address the problem of crosstalk by aligning the scintillator array with the photosensors very accurately. This alignment requires a lot of accuracy in the mechanical dimensions of a scintillator and also needs to meet difficult-to-achieve tolerance requirements of the photosensor-scintillator alignment. Further, some known methods correct only one particular type of crosstalk and are unable to correct the others. For example, boosted reconstruction kernels correct the average crosstalk between the elements of the detector, but do not correct the other types of crosstalks such as differential crosstalk. In addition, known methods do not provide any robust solution for correcting image artifacts in the CT images of objects that have sharp slopes.